The steps of preparing spun yarns according to the pneumatic spinning method are quite different from the steps of preparing spun yarns according to the ring spinning method except the step using a drafting device, and in the pneumatic spinning method, a drafted sliver is ballooned by using a fluid jet nozzle and twists are imparted when balloons are formed, and spun yarns are thus obtained.
Accordingly, the structure of a yarn obtained according to the pneumatic spinning method is different from the structure of a yarn obtained according to the ring spinning method. In the pneumatic spinning method, the properties of yarns, such as uniformity, strength and feeling, are greatly influenced by balloon factors such as the rotation number and diameter of balloons, and the pneumatic spinning method is inferior to the ring spinning method in the stability of the yarn properties.
Since the above-mentioned balloon factors are changed comprehensively by spinning conditions, for example, the nozzle structure (fluid jetting angle, inner diameter and the like), the fluid pressure, the spinning tension and the drafting unevenness, analysis of factors changing the ballooning state is very difficult, and even at the present, spinning conditions are independently determined in respective plants according to empirical laws while a method for analyzing these factors is not established.